1. Field of Invention
The invention relates to compositions and processes for depositing phosphate-containing conversion coatings, such as zinc phosphate and iron phosphate conversion coatings, on metal surfaces. In particular, the invention pertains to the use of substituted hydroxylamines as accelerators in zinc and iron phosphating processes and as dissolution aids in the preparation of manganese phosphate conversion coating compositions.
2. Statement of Related Art
The general process of phosphate conversion coating is well known in the art; such processes are commonly used to impart corrosion resistance to metal surfaces. Contacting active metals with aqueous acidic compositions containing zinc and phosphate ions, for example, results in the deposition on the active metal surfaces of a conversion coating containing zinc phosphate. If the active metal is ferrous, iron phosphates are usually included in the coating, and in modern practice nickel and/or manganese are often included in the coating composition and thereby in the coating formed. If alkali metal and/or ammonium cations rather than zinc cations are present in the coating composition, an iron phosphate conversion coating is formed on the metal surface. In order to speed the conversion coating process, lower the phosphating temperature, and/or improve the uniformity or quality of the coating, it is customary to include in the coating composition a component called an “accelerator” that does not usually become incorporated into the coating formed. Typical widely used accelerators include nitrate and nitrite ions, chlorate, soluble nitroaromatic organic compounds such as p-nitrobenzene sulfonic acid and hydroxylamine and its salts (e.g., hydroxylamine sulfate).
Although the most widely used accelerator in current commercial practice is believed to be nitrite, hydroxylamine and its salts have in recent years become more frequently employed in metal phosphating processes. Unfortunately, however, hydroxylamine is now under investigation as a possible carcinogen. If the carcinogenity of hydroxylamine is confirmed, this is likely to lead to stricter regulations and controls on the use and disposal of this substance and perhaps even a complete ban on its production and/or use in certain localities.
It would therefore be very desirable to develop alternative phosphating systems which use accelerators other than hydroxylamine but which provide conversion coatings of at least comparable quality.